Many charge coupled image sensing devices (CCDs), usually consisting of an array of pixels, require that light passes through the gate electrodes of the device before it is absorbed in the semiconductor substrate. To improve the sensitivity of such CCDs, transparent materials, such as indium-tin oxide (ITO), have been employed for one or more of the gate electrodes. Many devices, and, in particular, devices with all gates composed of ITO, employ Chemical Mechanical Planarization (CMP) processes in their fabrication (e.g. U.S. Pat. No. 6,300,160 or U.S. Pat. No. 6,403,993). A typical optical path for light to pass from air into the silicon substrate is shown schematically in FIG. 1. In FIG. 1 the silicon substrate 1, is covered with a gate dielectric, 2 a gate electrode 3, an overcoating dielectric layer 4 which has been subject to CMP processing, a passivation layer 5, and an anti-reflection coating 6. The thickness of layers 4 and 5 are indicated by dimensions 7 and 8.
In operation of an imaging device with such a structure, light must pass from the region above layer 6, through layers 6, 5, 4, 3, and 2, and be absorbed in the substrate 1. At each of the material interfaces of this structure, reflections can occur which produce optical interference which, in turn, leads to variations in the percent of light, as a function of wavelength, which penetrates into the silicon substrate 1. The degree of interference at any given wavelength is a complex function of the layer thicknesses and the indices of refraction of the various layers. Thus, if a given layer, 4 in this case, is of varying thickness across the array of pixels of the device, then the interferences will also vary across the array and thus the sensitivity of the device at any given wavelength will be varying. By way of example, FIG. 2 shows the calculated transmission of light into the silicon substrate as a function of wavelength for two different thicknesses of layer 4. In this figure, curves a and b represent the undulations in transmission of light into the silicon substrate for two thicknesses, respectively, of layer 4 which differ by 150 nm. It is evident that there is a significant difference in the transmission depending on the thickness of layer 4.
The processes and devices described in U.S. Pat. No. 6,300,160 or U.S. Pat. No. 6,403,993 disclose a device with all ITO electrodes which is fabricated using one or more CMP operations. Unfortunately, CMP inherently produces variations in the thickness of some of the layers overlying the gate electrodes. These layer thickness variations cause non-uniformities in the photoresponse of the devices. Such variation in photoresponse is undesirable. The present invention is directed toward improving the spectral response uniformity of image sensors wherein thickness of one or more of the constituent layers is non-uniform over the photosensitive area.